A steam turbine for generating utility power includes, inter alia, a steam chest where high pressure steam from a boiler or other steam source is collected and then admitted through apertures controlled by valves into the turbine casing, where its energy is utilized to rotate a power shaft or rotor. The steam chest is preferably located as close to the turbine as possible to minimize heat loss and pressure drops. Efficiency of the turbine increases with increasing temperature and pressure, but high pressures and temperatures involve inherent thermal stress problems that turbine designers must address. Turbine casings must be exceedingly strong to withstand high steam pressures. Turbine parts and ancillary equipment subjected to high temperatures must be free to expand and contract with temperature changes. Walls thick enough to withstand the high pressures involved can experience differential thermal expansion due to temperature gradients, resulting in high thermal stresses of the turbine casing and steam chest. The turbine and integral steam chest are subjected to severe thermal stresses during load cycling and serious cracking has occurred in various parts of the steam chest and steam turbine if care is not taken in the manner in which the steam is introduced into a cold turbine.
In general, the admission of steam to a steam turbine raises a significant problem of matching the temperature of the steam with the temperature of the turbine in order to avoid thermal stresses, particularly in the rotor. Efficiency of utilization of the steam and of the steam turbine requires that matching of such temperatures be achieved promptly in order to minimize the lag between a cold steam input during a restart and a hot turbine rotor, or between a hot steam input and a cold turbine rotor, both processes being necessary to minimize rotor stress in plant start-up time. Various systems have been developed for controlling the admission of steam into a steam turbine in a manner to minimize stresses on the turbine rotor during start-up or during cycling of the rotor between high and low power conditions. U.S. Pat. No. 4,589,255 assigned to the assignee of the present invention addresses the effects of thermal loading on a steam turbine and the risk of rotor thermal stress and plastic strain due to rapid thermal gradients placed upon the turbine.
While it has been recognized that the steam chest is also subjected to significant thermal stresses during cycling of the steam turbine, it is not believed that an adequate solution to minimizing the thermal stress on the steam chest has been developed. Prior art attempts to control steam chest thermal stresses have primarily relied upon intervention by an operator of the steam turbine relying solely on judgment to decide if the differential temperature between steam being introduced into the steam chest and the temperature of the steam chest is such as to avoid failure of the steam chest due to thermal stress. In some instances, such judgment has proven to be faulty. In these prior art systems, it is a general practice to close a set of control valves and modulate a throttle valve to allow some flow of high temperature steam into the steam chest. By controlling the flow into the steam chest, it is intended to produce a control ramp of steam chest metal temperature and thus reduce thermal fatigue. However, it is believed that such a process does not minimize thermal stress on the steam chest and in fact may introduce other thermal stresses on the chest.